The present invention relates to a method for forming carbon nanofibers, which are also known as nanotubes, by means of a generated plasma and chemically assisted catalysis.
Carbon fibers hold great promise as a high-performance material for use in composites due to their high strength and high modulus. They are commonly made by elevating a precursor material such as polyacrylonitrile (PAN) or pitch in an inert atmosphere to a temperature around 1000.degree. C. on continuous wind-up devices. The fibers formed by this process are generally continuous filaments and approximately 8 .mu.m in diameter.
Recently, carbon nanofibers have been produced directly from hydrocarbons in a gas phase reaction upon contact with a catalytic metal particle when heated by thermal energy to around 1000.degree. C. in a non-oxidizing gas stream. Carbon nanofibers differ physically from commercial carbon fibers in that they are thinner and are not continuous. Typically, they have a length around 0.01 mm to 0.1 mm and have a diameter generally of 10 nm to 100 nm. Carbon nanofibers also differ functionally from commercial carbon fibers in that they have a higher thermal conductivity, have high electrical conductivity, and are less subject to etching. Accordingly, carbon nanofibers offer great promise in a number of applications.
Carbon nanotubes are structures which consist of a sheet of carbon atoms in graphene forms wrapped into a cylinder. A single walled nanotube has only a single atomic layer. Multiwalled nanotubes have additional layers of carbon deposited pyrolytically. Multiwalled nanotubes may thus contain 100 to 1000 atomic layers. Multiwalled nanotubes have excellent strength, small diameter (typically less than 200 nm) and near-metallic electrical conductivity, making them useful as an additive to enhance structural properties of composites such as carbon-carbon, carbon-epoxy, carbon-metal, carbon-plastic, carbon-polymer and carbon-concrete.
Carbon nanotubes have been produced by arc-evaporation chemical reactions and by pyrolysis reactions using lasers to initiate pyrolysis. However, these methods are not effective for producing significant quantities of nanofibers. Thus, a need exists in the art for a method which can reliably and effectively produce carbon nanofibers in significant quantities.